Problem: Solve for $x$ and $y$ using substitution. ${-5x-y = -7}$ ${x = -4y+9}$
Answer: Since $x$ has already been solved for, substitute $-4y+9$ for $x$ in the first equation. ${-5}{(-4y+9)}{- y = -7}$ Simplify and solve for $y$ $20y-45 - y = -7$ $19y-45 = -7$ $19y-45{+45} = -7{+45}$ $19y = 38$ $\dfrac{19y}{{19}} = \dfrac{38}{{19}}$ ${y = 2}$ Now that you know ${y = 2}$ , plug it back into $\thinspace {x = -4y+9}\thinspace$ to find $x$ ${x = -4}{(2)}{ + 9}$ $x = -8 + 9$ ${x = 1}$ You can also plug ${y = 2}$ into $\thinspace {-5x-y = -7}\thinspace$ and get the same answer for $x$ : ${-5x - }{(2)}{= -7}$ ${x = 1}$